1. Field of the Invention
This invention relates to particular copolymers and terpolymers prepared in part from the monomer 2-acrylamido-2-methylpropane sulfonic acid (AMPS) which are at least water dispersible and their use as fluid loss control additives for aqueous drilling fluids which are used in drilling wells in subterranean formations.
2. Description of the Prior Art
Rotary systems are generally used in drilling oil and gas wells. These systems depend upon rotation of a string of drill pipe to the bottom of which is attached a drill bit. The bit cuts into the formation causing the cuttings to accumulate as drilling continues. A drilling fluid is used which lubricates the drill bit and carries cuttings to the surface for removal. This allows the bit to continue functioning and the bottom hole to be clean and free of cuttings. The drilling fluid is also the source of pressure which is exerted against the formation. Even in other drilling systems, drilling fluids are still needed to remove bore hole cuttings and to otherwise perform functions related to drilling fluids.
Oil and gas producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. When drilling throgh such porous formations, it is essential to employ drilling fluids having characteristics such that excessive amounts of the drilling fluid do not penetrate the porous formation. Drilling fluids have a tendency to penetrate the formation because the pressure in the bore hole is greater than the pressure opposing it which comes from the formation. Should excess penetration occur, there is loss of drilling fluid into the formation resulting in loss of pressure, inability to drill and remove cuttings and in the case of loss from brine muds, extra expense because of their cost. Mere filter cake formation on the wall of the bore hole does not adequately eliminate fluid loss.
Various materials have been added to drilling fluids to control fluid loss and have included materials such as pregelatinized starch, sodium carboxymethyl cellulose, sodium polyacrylates and lignites.
U.S. Pat. No. 2,718,497--Oldham et al, Sept. 20, 1955, disclose that polymers of acrylic acid when added to drilling muds reduce fluid loss of same. The muds to which the polymer is added is added do not contain significant quantities of soluble calcium or chloride ions.
U.S. Pat. No. 4,293,427--Lucas et al, Oct. 6, 1981 describe copolymers of (1) a (meth)acrylamido alkyl sulfonic acid, including AMPS, and (2) a (meth)acrylamide or N-alkyl (meth)acrylamide or cross-linked copolymer thereof as filtration control agents for drilling fluids in subterranean formations. Other filtration control agents for drilling fluids are referred to as well.
U.S. Pat. No. 4,309,523--Engelhardt et al, Jan. 5, 1982 disclose drilling mud additives which are polymers of AMPS and N-vinylamides. Water loss properties are measured.
U.S. Pat. No. 3,547,899--Arit et al, Dec. 15, 1970 disclose homopolymers and copolymers of AMPS. Acrylic acid is disclosed as a comonomer. The polymers may be used to prepare films and fibers.
U.S. Pat. No. 3,806,367--Lange et al, Apr. 23, 1974 and U.S. Pat. No. 3,898,037 --Lange et al, Aug. 5, 1975 disclose copolymers and terpolymers of AMPS in molecular weight ranges of 750 to 5,000,000, the preferable range being from about 750 to 250,000. Comonomers are vinyl monomers with acrylic acid and itaconic acid enumerated among others. These polymers are used as dispersants for particles in aqueous systems.
U.S. Pat. No. 3,928,196--Persinski et al, Dec. 23, 1975 disclose a method of inhibiting scale formation using copolymers containing about from 5 to 75 mole % of AMPS and from about 95 to 25 mole % of acrylic acid having molecular weights of from about 1,000 to 100,000.
U.S. Pat. No. 4,015,991--Persinski et al, Apr. 5, 1977 disclose copolymers of AMPS and acrylamide hydrolyzed to the extent of at least 20% as fluid loss additives for cement.
U.S. Pat. No. 4,432,879--Greaves et al, Feb. 21, 1984, disclose deposition prevention agents for cooling water systems which are combinations of (1) particular phosphonates and (2) water soluble copolymers. The copolymers are vinyl addition type copolymers where a copolymer of methacrylic acid and AMPS in a 1:1 mole ratio and 5,000 molecular weight is preferred. Acrylic acid is enumerated among other ethylenically unsaturated acids and anhydrides. Mole proportions of the monomers is generally from 5:95 to 95:5, preferably 50:50 and molecular weight ranges from 500 to 750,000, in particular from 1,000 to 10,000 and especially from 4,000 to 6,000.
Where polyacrylates are added to fresh water muds, fluid loss can be reasonably controlled. However, where soluble calcium or chloride ions are present in the muds, generally in excess of about 300 ppm of soluble calcium ions and in excess of about 1,000 ppm of chloride ions, as for example from the cement or from the formation in the case of the former and from seawater (brine) and salt water in the case of the latter, particularly under conditions of high temperaure and pressure, polyacrylates are not as effective in controlling fluid loss.